Preparation and visible light photocatalytic performance of methylene blue intercalated K4Nb6O17

ARTICLE IN PRESS Journal of Physics and Chemistry of Solids 71 (2010) 35–41

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Preparation and visible light photocatalytic performance of methylene blue intercalated K4Nb6O17 Wenwu Qu, Feng Chen n, Bin Zhao, Jinlong Zhang Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China

a r t i c l e in fo

abstract

Article history: Received 27 March 2009 Received in revised form 10 September 2009 Accepted 9 October 2009

K4Nb6O17 was prepared by hydrothermal treatment of Nb2O5 in KOH solution at 180 1C, and then Methylene blue (MB) intercalated K4Nb6O17 (K4Nb6O17-MB) was prepared by one-pot reaction in which n-propylamine (PA) was used as an intercalation compound. The MB intercalated structure of K4Nb6O17MB was characterized by HRTEM and XRD measurements. K4Nb6O17-MB shows good absorption in the visible region and is thermally stable up to 328 1C. By extending the hydrothermal time and selecting the K4Nb6O17 with high crystallinity, the K4Nb6O17-MB prepared by one-pot reaction showed higher visible light (l 4 550 nm) photocatalytic activity than that prepared by traditional two-step electrostatic self-assembly deposition (ESD) method for the degradation of methyl orange (MO). & 2009 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures A. Inorganic compound C. Electron microscopy C. X-ray diffraction

1. Introduction Layered oxide materials have potential applications in various fields such as electrocatalysis, photocatalysis, energy-storage and energy-conversion [1,2]. Recently, much attention has been attracted to enhance the above performances of layered oxide materials by intercalating with metal compounds [3,4] (such as ZnS and TiO2) or organic molecules [5,6] (such as dye). Among the layered oxide materials, potassium hexaniobate K4Nb6O17 has got the focus because of its distinctive layered structure. K4Nb6O17 possesses two alternative interlayer spaces formed by repeating [Nb6O17]4  layers between which K + exists to hold the layers together and to maintain the charge balance. Because of its unique layered structure, the photocatalytic properties of K4Nb6O17 have been widely studied [7,8]. It has been reported that 100 mg/L acid red G aqueous solution was smoothly degraded by K4Nb6O17 powder in 1 h under the UV irradiation of a commercial 20 W UV lamp [8]. Much effort has also been made to improve the photocatalytic performance of K4Nb6O17, most of which involved intercalation. Various cations were intercalated into the [Nb6O17]4  layers of K4Nb6O17 to efficiently improve the photocleavage of water [9–11]. Since the host layered oxide always has a very large band-gap (3.54 eV for K4Nb6O17 in our work), their photocatalytic processes only take place under the UV light irradiation. Thus, how to make full use of

n

Corresponding author. Tel./fax: + 86 21 64252062. E-mail address: [email protected] (F. Chen).

0022-3697/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2009.10.005

the visible light which takes the most proportion of solar light has got the recent concerns. Several modifications have been carried out in the interlayer or on the surface of the host layered oxide [12,13] to utilize the visible portion of the readily available sunlight. Kim [14] and Inoue [15,16] have reported dyes or complexes intercalated K4Nb6O17 materials working properly under the visible light. An electron transfer which takes place between the photoexcited dye (or complex) and the conductive band of the host K4Nb6O17 under visible irradiation was suggested to provoke the reaction. Because of the mass transfer limitation, a whole intercalating process for layered oxides always takes several days and even weeks [17–19]. For the sake of reducing the total intercalation time, electrostatic self-assembly deposition (ESD) technique was developed, which provides a quick and reliable way for introducing complex molecules into the interlayer [20]. However, traditional ESD method usually contains two steps: an exfoliation (or pre-intercalation) step and an intercalation step. And for traditional ESD method the exfoliation step usually needs acidification pre-treatment which transforms K4Nb6O17 to H4Nb6O17[18]. In this work, ESD technique was improved and utilized to intercalate methylene blue (MB) molecules into the interlayers of K4Nb6O17 by one-pot reaction, of which the two separated steps of the traditional ESD technique were combined into one step. n-propylamine (PA) was utilized as the exfoliation reagent without acidification pre-treatment. The photocatalytic performances of MB intercalated K4Nb6O17 were estimated by the degradation of methyl orange (MO) aqueous solution under the visible light irradiation.

ARTICLE IN PRESS W. Qu et al. / Journal of Physics and Chemistry of Solids 71 (2010) 35–41

2.4. Characterization The crystalline phases of the products were identified by powder X-ray diffraction (XRD) with a Rigaku D/max 2550 VB/PC X-ray diffractometer at room temperature using Cu Ka radiation (l = 0.154056 nm) and a graphite monochromator, operated at 40 kV and 100 mA. TEM and HRTEM images were taken with a JEOL JEM-100 CX II transmission electron microscope at an acceleration voltage of 100 kV. The absorption edges of the products were determined from UV–vis diffuse reflectance spectra (DRS) using a Scan UV–vis–NIR spectrophotometer (Varian Cary 500). The weight loss and thermal stability of K4Nb6O17-MB were determined by TG–DTA measurements (PerkinElmer Pyris Diamond TG/DTA).

XRD patterns of the samples prepared by Nb2O5 in the different concentrations of KOH solution are shown in Fig. 1. The concentrations of KOH used for the K4Nb6O17 preparation were 0.36, 0.50, 0.75, 1.00 and 3.50 M, respectively, which were corresponding to 0.67, 0.93, 1.39, 1.85 and 6.48 times of the amount of Nb atoms and a very alkaline reaction environment. When the concentration of KOH was fixed at 0.36 M, a ratio of K:Nb=2:3, the product was Nb2O5 but not K4Nb6O17. In this system, KOH not only serves as a reactant but also as the reaction medium. It seems that an concentration of 0.36 M OH  is not alkaline enough to dissolve Nb2O5 powders and to transform it into K4Nb6O17. When the amount of KOH was raised and reached to 0.50 M, K4Nb6O17 was identified in XRD pattern as the main product; however, the peaks at 22.611, 28.361 and 28.961 showed the existence of Nb2O5 crystals residue, which showed that KOH here was not yet enough for the total crystallization of K4Nb6O17. When the concentration of KOH was increased to 0.75 and 1.00 M, K4Nb6O17 signals were obtained without any Nb2O5 impurity. As compared to the JCPDS 21-1295 and JCPDS 21-1296, it can be inferred that the hexaniobate was an orthorhombic K4Nb6O17  nH2O (n E4.5) with the crystal cell parameters a= 0.771 nm, b=4.135 nm and c= 0.642 nm. The corresponding crystal face indexes of the peaks are labeled on the curve e in Fig. 1. From the peak intensity we can see that the crystallinity of

f (1(16)0)

As-prepared K4Nb6O17 powder was firstly added into the water. Then desired amounts of MB and PA were added under stirring. The molar ratio of MB/PA/K was fixed at 0.75:1.00:1.00. The suspension was then put into a 100 mL autoclave and allowed to react at 120 1C for 24 h or 72 h. After being cooled, the suspension was filtrated and carefully washed with ethanol until the filtrate became colorless to remove the adsorbed MB on the surface. The final obtained composite was then signed as K4Nb6O17-MB. A controlled experiment was also carried out to investigate the exfoliation effect of PA. In this case, only PA and K4Nb6O17 were added into water and hydrothermally treated at 120 1C (for 24 h or 72 h). Then MB was added and the suspension was stirred for another 12 h under room temperature. The suspension was filtrated and carefully washed with ethanol. The obtained intercalated composite was signed as K4Nb6O17-MB0 . Finally, the obtained K4Nb6O17-MB (or K4Nb6O17-MB0 ) was dried in vacuum furnace at 60 1C for 6 h for use.

3.1. XRD and TEM analysis of K4Nb6O17

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2.3. Preparation of MB intercalated K4Nb6O17 (K4Nb6O17-MB) with one-pot reaction

3. Results and discussion

(270)

K4Nb6O17 was prepared according to the procedure described by Li [21]. A desired amount of KOH, 5.0 g Nb2O5 and 70 mL water were mixed and put into a 100 mL autoclave, heated up to 180 1C to provoke a hydrothermal reaction for 72 h. Then a solid powder was separated from the aqueous solution by filtration. It was rinsed repeatedly with water and dried by infrared light overnight for further use.

(200)

2.2. Synthesis of K4Nb6O17

(060)

MB, MO and Nb2O5 were of AR and obtained from Sinopharm Chemical Reagent Co. Ltd. KOH (AR) was obtained from Lingfeng Chemical Reagent Co. Ltd. PA (AR) was obtained from Johnson Matthey Co. All the reagents were used as received. Doubly distilled water was used throughout the work.

(040)

2.1. Materials

visible light irradiation was carried out with a 1000-W halogen lamp. To keep the reaction temperature constant, cooling water in a quartz cylindrical jacket round the lamp was used. A cut-off optical filter was placed in front of the quartz cylindrical jacket to entirely remove the light with wavelengths below 550 nm. The distance between the light source and the reaction quartz tube was fixed at 20 cm. At every given time interval, a sample of 3.5 mL suspension was withdrawn, centrifuged and filtered. The residual concentration of MO aqueous solution in the solution was measured with a UV–vis spectrophotometer (Varian Cary 100) at a wavelength of 464 nm, which is the maximum absorption of MO aqueous solution.

(020)

2. Experimental

Intensity (a.u.)

36

e

d c × 0.2

b × 0.2 2.5. Photocatalytic activity measurement The photocatalytic activity of the samples was measured in terms of the degradation of MO aqueous solution. For each measurement, 70 mg of photocatalyst sample was added into a 160 mL quartz tube containing 70 mL of 10 mg/L MO aqueous solution. Before irradiation, the suspension was stirred in the dark for 30 min to attain the adsorption–desorption equilibrium for MO and dissolved oxygen on the surface of the photocatalyst. The

a 5

10

15

20

25

30

35

40

45

2θ (degree) Fig. 1. XRD patterns of (a) Nb2O5, and the samples prepared from Nb2O5 with (b) 0.36 M, (c) 0.50 M, (d) 0.75 M, (e) 1.00 M and (f) 3.50 M KOH solutions (the solid powder for the curve f was obtained by neutralizing the alkali with HCl).

ARTICLE IN PRESS W. Qu et al. / Journal of Physics and Chemistry of Solids 71 (2010) 35–41

K4Nb6O17 which was prepared in 1.00 M KOH (curve e) was better than that in 0.50 M KOH (curve d). Several literatures [22,23] reported that the generation of K4Nb6O17 needs 250 1C or higher temperature and even subcritical and supercritical water conditions; while Li [21] reported that the K4Nb6O17 crystal was obtained through hydrothermal treatment in 0.50 M KOH aqueous solution at 180 1C for 96 h. The XRD results here showed that K4Nb6O17 can be obtained at 180 1C for 72 h in 0.50 M KOH. However, according to our results, the transformation of Nb2O5 to K4Nb6O17 remained incomplete. Surely, due to the reaction loss of KOH and to obtain a perfect K4Nb6O17 product, a KOH concentration of 0.75 M would be better. When the concentration of KOH aqueous solution was increased to 3.50 M, no solid product was observed in the autoclave because KOH solution of high concentration (3.50 M) dissolved the Nb–O clusters without crystallization. Neutralization of the alkali with HCl can give some solid powder which was amorphous (curve f). As a result, the concentration of KOH is a crucial factor in the hydrothermal preparation of K4Nb6O17. The TEM images of the samples obtained in various KOH solutions are shown in Fig. 2. TEM images present clearly that the crystal K4Nb6O17 was composed of layered structure and of uniform square (Figs. 2b–d). The size of K4Nb6O17 obtained in 0.50, 0.75 and 1.00 M KOH solution was ca. 68, 73 and 114 nm, respectively. Some impurities appeared in the TEM image (Fig. 2b) of K4Nb6O17 obtained in 0.50 M KOH should be due to Nb2O5 residue. The morphology of the particles in Fig. 2a is obviously different from those in Figs. 2b–d. In according with the XRD results in Fig. 1, the solid obtained in 0.36 M KOH was Nb2O5 but not K4Nb6O17.

37

3.2. HRTEM and XRD analysis of K4Nb6O17-MB K4Nb6O17 possesses a couple of interlayer environments (named interlayer I and interlayer II) with different reactivities [24,25]. The potassium ions in interlayer I can be easily exchanged while those in interlayer II are relatively difficult to be replaced. Some previous reports suggested that only interlayer I of K4Nb6O17 can be intercalated by tris(2,2-bipyridine) ruthenium(II) ions [25] and methylviologen [26] via the traditional intercalation method. Fig. 3 presents the HRTEM images of K4Nb6O17-MB. The HRTEM images show the basal spacing of 2.26 nm for K4Nb6O17MB prepared at 120 1C for 72 h. The white stripes should be the definite position of interlayer MB—with a space of 1.44 nm, which is just the gallery height of MB in the interlayer. MB molecules were clearly observed in both interlayers, which supported the previous reports of Ogawa [20] and Unal [27]. Thus, it can be concluded that K4Nb6O17-MB prepared in this work was of B-type intercalation [28]. ESD technology exfoliates effectively the layers of K4Nb6O17, which ensures the intercalation of MB into each interlayer. Fig. 4 represents the XRD patterns of K4Nb6O17 and various kinds of K4Nb6O17-MB. The basal spacing of various samples can be calculated from the XRD peaks of (0 2 0) crystal face. It has been reported that the thickness of each niobate host layer for dehydrated K4Nb6O17 is 0.82 nm [20]. It can be clearly observed that the basal spacing of the samples in Fig. 4 was greatly expanded as compared to that of K4Nb6O17 (curve a). Calculated from the XRD peak of (0 2 0) crystal face, the basal spacing for the K4Nb6O17-MB (curve c) is 2.26 nm. Therefore, the gallery height of the MB intercalated in the interlayer should be

Fig. 2. TEM images of the samples obtained from Nb2O5 hydrothermally treated with (a) 0.36 M, (b) 0.50 M, (c) 0.75 M and (d) 1.00 M KOH solutions.

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560nm

1.5

(060)

Intensity (a.u.)

(040)

(020)

516nm

Fig. 3. HRTEM images of K4Nb6O17-MB prepared at 120 1C for 72 h.

d c

547nm

Abs

e 1.0

c b

0.5 d

b

a 0.0

a

300

5

10

15

20

25 30 2θ (degree)

35

40

45

Fig. 4. XRD patterns of (a) K4Nb6O17 and K4Nb6O17-MB prepared for (b) 24 h and (c) 72 h, K4Nb6O17-MB0 prepared for (d) 24 h and (e) 72 h.

2.26–0.82=1.44 nm which is consistent with the HRTEM measurement in Fig. 3. Taking the molecular size of MB (ca. 1.40 nm  0.65 nm  0.40 nm) [29] into consideration, the intercalated MB is thought to have a bilayer with their molecular planes parallel to the host niobate layers or to form a monolayer with their molecular plane and the longer axis perpendicular to the host niobate layers [20,27]. The hydrothermally treated K4Nb6O17-MB sample with MB for 24 h (curve b) gave a similar XRD pattern with that of K4Nb6O17-MB sample hydrothermally treated for 72 h (curve c). It means the intercalation process mostly proceeded in the first 24 h (however, obvious differences in photocatalytic activities were observed between these two K4Nb6O17-MB samples given in Fig. 7). The basal spacing of controlled samples of K4Nb6O17-MB0 was calculated as 2.76 nm (curve d) and 2.81 nm (curve e), which was larger than that of K4Nb6O17-MB (2.26 nm). It seems that traditional two-step ESD method (controlled experiments) can intercalate more MB molecules into the interlayer of K4Nb6O17. Surprisingly, for the K4Nb6O17-MB0 prepared with traditional ESD method, that is to say, K4Nb6O17 pre-intercalated with PA (curves d & e), the calculated crystal distances for (0 2 0), (0 4 0) and (0 6 0) did not obey the ratio of 1/2:1/4:1/6 strictly. It means

400

500 600 Wavelength (nm)

700

800

Fig. 5. DRS UV–vis spectra of (a) K4Nb6O17, (b) K4Nb6O17-MB, (c) physically mixed K4Nb6O17 and MB via grinding and (d) K4Nb6O17 surface adsorbed with MB.

the periodicity of niobate host layers was partially disobeyed. In other words, the interlayer space of interlayers I and II was possibly different for the K4Nb6O17-MB0 . Therefore, the diffraction peaks of (0 2 0), (0 4 0) and (0 6 0) crystal faces are interfered with the diffraction of (0 3 0) and (0 5 0) crystal faces (Because of the diffraction counteraction, the diffraction peaks of (0 3 0), (0 5 0) would not appear for the layered materials with a strict periodicity.) 3.3. DRS UV–vis spectra and TG–DTA analysis of K4Nb6O17-MB DRS UV–vis spectra of K4Nb6O17, K4Nb6O17 after MB adsorption, K4Nb6O17-MB and physically mixed (via grinding) K4Nb6O17 and MB are shown in Fig. 5. The absorption edge of K4Nb6O17 crystal was around 350 nm (curve a), which is corresponding to an energy band-gap of 3.54 eV and showed no response to visible photons. K4Nb6O17-MB showed a very strong and broad absorption under visible light region (curve b), which showed a possible photocatalytic ability of the K4Nb6O17-MB in the visible light region. A simple physical mixture of K4Nb6O17 and MB also showed absorption in visible region, with a maximum absorption around 560 nm (curve c). The maximum absorption of K4Nb6O17MB was shifted to 516 nm (Dl = 44 nm). K4Nb6O17 surface adsorbed with MB (curve d) has weaker absorption intensity

ARTICLE IN PRESS W. Qu et al. / Journal of Physics and Chemistry of Solids 71 (2010) 35–41

14

Light On

a b c

1.0

100

8 6

90

4

0.8 d C/C0

10 DTA

TG

Heat Flow (mW)

12

95 Weight (%)

39

0.6

e

0.4

f g

85

2

0.2

0 100

200

300 400 Temperature (°C)

500

Fig. 6. TG/DTA curves of K4Nb6O17-MB prepared at 120 1C for 72 h.

than K4Nb6O17-MB and a maximum absorption at 547 nm. Surely, MB is connected with the [Nb6O17]4  host layer by the chemical but neither surface adsorption nor simple physical interaction. The interaction of MB with K4Nb6O17 is much stronger in K4Nb6O17-MB than that in the surface adsorption. MB is a cation dye, has higher polarity at ground state; therefore, a strong chemical interaction with the negative niobate layer would stabilize more the ground state of MB, which enlarged the energy difference between its excited-state and ground state, resulting in an obvious blue-shift in UV–vis absorption for MB in K4Nb6O17-MB. The TG/DTA curves of K4Nb6O17-MB are shown in Fig. 6. The weight loss below the temperature of 110 1C was because of the removal of free or physically adsorbed water, while that between the temperatures of 110 and 328 1C was attributed to the subsequent removal of interlayer bonded water molecules and residuary PA molecules. A little exothermic peak at 258 1C was probably attributed to the burning of residual PA in the interlayer. The decomposition and removal of MB in the interlayer started at 328 1C, which gave a weight loss of 9.7% between 328 and 570 1C. Correspondingly, a main exothermic peak at 382 1C was raised because of the decomposition of MB molecules in the interlayer. Mass loss of intercalated MB with a value of 9.7% is corresponding to a MB:[Nb6O17]4  molar ratio of 0.30, which is similar with the results previously reported in Nakato’s paper [28]. Although the maximum calculated amount of [M(bpy)3]2 + per [Nb6O17]4  can reach 0.55 for B-type intercalation, the molar ratios of various [M(bpy)3]2 + in K4Nb6O17 were only 0.3, while those for the A-type intercalation varied from 0.1 to 0.2 [28]. 3.4. Photocatalytic activity and mechanism Fig. 7 presents the visible light induced photocatalytic degradation of MO with the various kinds of photocatalysts. K4Nb6O17 which was prepared in 0.75 and 1.00 M KOH was named KNbO-1 and KNbO-2, respectively, for convenience. The concentration changes of MO before the light-on were due to the dark adsorption of MO on the surface of photocatalysts. The controlled experiment showed that little bleaching was observed for MO in the absence of photocatalysts (curve a). K4Nb6O17 itself can hardly degrade MO under visible light irradiation (n.b. all the photons of wavelength shorter than 550 nm were cut off by the optical filter), and only a negligible absorption loss appeared for 7.0 h irradiation (curves b and c). The intercalation of MB endowed remarkable photocatalytic activities to the photocatalysts, which is due to the electron transfer between

0.0 0

1

2 3 4 Irradiation Time (h)

5

6

7

Fig. 7. Visible light induced photocatalytic degradation of MO (a) in homogeneous solution, with (b) KNbO-1, (c) KNbO-2 and with (d) K4Nb6O17-MB prepared from KNbO-1 for 24 h, (e) K4Nb6O17-MB prepared from KNbO-2 for 24 h, (f) K4Nb6O17MB prepared from KNbO-1 for 72 h and (g) K4Nb6O17-MB prepared from KNbO-2 for 72 h.

the excited MB and the host layers of K4Nb6O17 under visible irradiation. K4Nb6O17-MB obtained by hydrothermal treatment for 72 h (curves f and g) had much better photocatalytic activities than those of K4Nb6O17-MB obtained for 24 h (curves d and e). It suggested that longer hydrothermal treatment was better for entire exfoliation of K4Nb6O17 and the intercalation of MB. K4Nb6O17-MB prepared from KNbO-2 (curves e and g) had higher activities than those of K4Nb6O17-MB prepared from KNbO-1 (curves d and f), probably because KNbO-2 had better crystallinity which was shown in Fig. 1. Fig. 8 presents the visible light induced photocatalytic degradation of MO with several K4Nb6O17-MB and K4Nb6O17MB0 . As shown in Fig. 8A, the K4Nb6O17-MB0 photocatalysts (curves c and d) presented higher photocatalytic activities than those of K4Nb6O17-MB photocatalysts (curves a and b) which were produced by one-pot reaction. It was mostly because the method with PA pre-intercalation is more beneficial for K4Nb6O17 exfoliation. However, with the extension of hydrothermal treatment time and the improvement of the crystallinity of raw K4Nb6O17 materials, the photocatalytic performance of K4Nb6O17MB was improved more effectively than that of K4Nb6O17-MB0 . Interestingly, K4Nb6O17-MB even shows a higher photocatalytic activity than K4Nb6O17-MB0 as shown in Fig. 8B. The adverse effect of the MB during the pre-intercalation can be eliminated by extending the hydrothermal time and selecting the raw K4Nb6O17 materials of high crystallinity. As a result, K4Nb6O17-MB that was prepared by hydrothermally treating KNbO-2 at 120 1C for 72 h through one-pot method showed the best photocatalytic activity and degraded 78% MO in 7.0 h under visible light irradiation. Mixed MB solution (2  10  5 M) with K4Nb6O17 resulted an entire adsorption of MB on K4Nb6O17. Irradiating this suspension with visible light led to a completely bleach of MB (on the surface of K4Nb6O17) in 80 min under the visible irradiation. Thus, the photostability of K4Nb6O17-MB was checked by observing the photocatalytic reactivity of recycled K4Nb6O17-MB for repeated degradation of MO. UV–vis spectroscopic measurement of the filtrate indicated MB molecule did not dissociate from the K4Nb6O17-MB photocatalyst, as no MB absorption (660 nm) was observed during all the photocatalytic experiments. Further, only a slight variation was presented on the degradation rate of the MO between the K4Nb6O17-MB and recycled K4Nb6O17-MB: the

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1.0

suspension at the beginning, and needs less reaction period than traditional ESD methods. The pre-intercalation of PA and the exfoliation of K4Nb6O17 can be achieved by mixing the K4Nb6O17 crystal and PA for 72 h at 120 1C, as reported previously by other researchers [30]. The coexistence of PA provided an enough large space for MB molecules intercalated into the [Nb6O17]4  host layers and thus to prepare the K4Nb6O17-MB with high visible light photocatalytic activity.

Light On

C/C0

0.8 a

0.6

c b

0.4

4. Conclusion

d The K4Nb6O17 crystals were prepared by hydrothermal treatment of Nb2O5 in KOH solution at 180 1C. The concentration of KOH was found as a crucial factor in the preparation of K4Nb6O17. The intercalation of MB into the interlayer of niobate layered oxide was achieved with ESD method by one-pot reaction. MB was intercalated into both interlayers (interlayer I and interlayer II) of niobate layered oxide. Intercalated compound K4Nb6O17-MB had good absorption in the visible light range, which was due to the presence of dye molecules in the interlayer. Therefore, K4Nb6O17MB degraded MO solution effectively under the visible light irradiation. The visible light photocatalytic activities of K4Nb6O17MB prepared by one-pot reaction can be effectively improved by extending the hydrothermal time and selecting the K4Nb6O17 with high crystallinity. By using the K4Nb6O17 prepared with Nb2O5 in 1.00 M KOH solution, the visible light photocatalytic activities of K4Nb6O17-MB prepared by one-pot reaction at 120 1C for 72 h was even higher than that of K4Nb6O17-MB0 prepared by traditional two-step ESD method.

0.2

0.0 1.0

Light On

C/C0

0.8

0.6

0.4 d b

0.2

0.0

Acknowledgment

0

1

2 3 4 Irradiation Time (h)

5

6

7

Fig. 8. Visible light induced photocatalytic degradation of MO with K4Nb6O17-MB prepared from (A) KNbO-1 and (B) KNbO-2 for (a) 24 h and (b) 72 h and K4Nb6O17MB0 prepared for (c) 24 h and (d) 72 h.

This work was supported by the National Nature Science Foundation of China (20777015) and Shanghai Nature Science Foundation (06ZR14025).

References degradation rates of MO in 7 h were in a region of 74.0 73.8% (77.7%, 73.6%, 70.2%, 71.4% and 72.5% for the as-prepared, once recycled, twice recycled, three times recycled and four times recycled K4Nb6O17-MB, respectively). Thus, MB is relatively stable under visible light irradiation after it was intercalated into the K4Nb6O17 layers, and can be recovered during the visible light photocatalysis of K4Nb6O17-MB. Under visible light illumination, MB molecule intercalated into the interlayer is excited to form MBn, which would transfer an electron from its HOMO to the conduction band of the [Nb6O17]4  host layer. The conduction band electron then moves and reacts with the surface adsorbed oxygen to execute the photocatalytic reaction [27]. The whole photocatalytic process is briefly shown as follow: MB+ hv-MBn

(1)

MBn-MBn + + e 

(2)

O2 + e  -Od2  + H + -HOOd--HOd

(3)

The intercalation reaction with PA as pre-intercalation reagent is very effective and convenient, and does not need acid treatment before the exfoliation as in conventional method [18,19]. The onepot reaction directly mixes the intercalation compound into the

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